Best practices for feature detection of DOM API

[alice]

One example: WebXR have a feature detection method, supportsSession(), which returns a promise which rejects if the session type is not supported.

Should this return a Promise<boolean>, on the principle that feature detection shouldn't throw?

Also, what other best practices are there for feature detection?

cc @torgo

[annevk]

https://www.w3.org/2001/tag/doc/promises-guide/#rejections-should-be-exceptional applies here I think. In particular, this does not seem exceptional.

[torgo]

We have some text in section 2.3 (New Features should be Detectable) so if we have more to say on this topic it should probably be added here.

[kenchris]

I think there is some overlap between feature detection and fingerprinting (and private browsing mode).

Of the top of my head I have heard different recommendations at different times.

Examples:

• don't expose the objects etc when feature is not available (like no underlying hardware gyroscope for instance)
• always expose (due to avoiding fingerprinting) but let object throw (NotAvailableError or similar) when instantiated - this way you can see whether the browser support the feature or not, but not whether the hardware does

All of this leads to different design and confuses users and spec authors. I am not sure what would be the right answer but it deserves some thought and it also requires some research on what kind of different solutions are in use today.

@anssik @slightlyoff @reillyeon @beaufortfrancois

[torgo]

Suggest having a dedicated breakout session on this during the non-design-review week next month.

[kenchris]

Things to keep in mind

• no diff between regular mode and privacy preserving mode (incognito)
• keeping fingerprinting to the minimum (people can know browser from UA string and features available)
• this involves permissions

Current different ways (non-exaustive):

• Presence of a namespace

if ('bluetooth' in navigator)

• Presence of an object

if ('NFCReader' in window)

• properties on objects, eg. Media Capture

const capabilities = videoTrack.getCapabilities();

if ("pan" in capabilities) { ... }

[torgo]

@triblondon do you have opinions here?